In recent years grinding has gained its significance as a stock removal process for shaping and sizing both hard and soft materials in contrast to what was realized in the past as a metal finishing operation and a process to be worked on very hard materials. To meet the ever increasing demand for improved productivity in the field of grinding, various new techniques have been introduced and are being investigated. These processes basically intended to provide for high rate stock removal, with improved work piece quality and prolonged grinding wheel life.
One of the most important requirements to be satisfied by the grinding wheels is free cutting action. This necessitates availability of large chip clearance volume ahead of individual crystals which clearly indicates that the gap between the adjacent grits in the direction of cutting should be wide enough. At the same time the protrusion of the grit above the bond should be large enough. Along with these conditions another requirement to be fulfilled is that the bond between grits and matrix should be strong to retain the grit throughout its useful life. The bonding material should have desirable mechanical properties like strength, hardness, low adhesion, low solubility in the ground material, and resistance to yielding during actual grinding.
The most cost effective method of preparing such a working surface of a wheel is the well known galvanic process. The galvanic process consists of interlocking the abrasive particles by galvanic deposition of metal like Ni or Co on a steel support (steel hub or steel mandrel). To date galvanic wheels with steel mandrels are the only type that can be used at a very high speed for safety reasons. But such wheels also suffer from several weaknesses. For effective retention of the grits in the bond, the grits are covered up to 60-80% of their height thus reducing the space for chip disposal. In addition, lump formation of material in the form of nodules is a common feature in the surface of such tools which further reduces the space in between the adjacent grits. Thus the galvanically bonded tool often fails to offer free cutting action.
It is understandable that effective bonding can be made even with less amount of binder if it is chemical or metallurgical in nature as obtained in liquid phase bonding or brazing. Such a process is disclosed in U.S. Pat. No. 4,239,502. A suitable process is to first coat the CBN particles with a silver base alloy, coat the surface of the rim portion of metal wheel and then clamp the coated CBN particles to the coated rim while applying heat to bond. The main shortcoming of the process is the complicacy in practicing the process of grit coating and obtaining separated grits. It is important that grits must be separated from one another before brazing and should not remain as a cluster of grits. Moreover, metal coated grits will generate large force and temperature as a result of metal-metal contact during grinding.
Methods of applying and metallurgically bonding hard carbide particles on the surface of a metal substrate are disclosed in U.S. Pat. Nos. 3,868,235; 3,378,361; 3,615,309; and 3,248,189. The bonding materials are mainly Ni-Cr base alloy containing other elements like Fe, B, Si and in most cases can be obtained commercially. It has been found that inclusion of Mo and Co in these alloys improves the bond provided by the matrix (U.S. Pat. No. 3,248,189). Cobalt-chromium-tungsten type "Stellite" alloys are also suitable (U.S. Pat. No. 3,615,309).
Diamond abrasive tools with a monolayer of abrasive grits, and a method of their manufacture are disclosed in U.S. Pat. Nos. 3,894,673 and 4,018,576. The methods utilize readily available, very hard and durable brazing alloys which are found to readily wet untreated diamond surfaces.
Another Pat.(No. PCT/US83/01946) teaches a method of making a monolayer abrading tool with tungsten carbide particles. A tape of powdered brazing material blended into a soft, flexible matrix is first secured to a steel substrate. Later the abrasive particles are imbedded to the tape. The tool is heated to a temperature at least equal to the liquidus temperature of the brazing material to set the abrasive particles and then the tool is cooled rapidly to solidify the brazing material to produce a metallic matrix.
The shortcoming of such tools is that they cannot be used for grinding steel because of vigorous reactions between the work material and the abrasive particles causing rapid wear. In addition, the brazing materials disclosed in the above-mentioned patents are apparently limited to the bonding of hard carbide and diamond particles. Nothing is disclosed on joining of CBN by the processes disclosed or materials used.
The present investigators have found that the Ni base alloys containing Cr which braze effectively uncoated diamond below 1200.degree. C. cannot provide a strong bond for CBN. The grinding experiments showed that CBN tools fail because of premature grit dislodgement due to poor adhesion between the bond and grit surface. The situation does not change when the amount of Cr in the alloy is raised to 25%. Thus it can be realized that Ni-Cr alloys cannot braze CBN grits because of the fact that Cr cannot effectively act as a wetting agent or react with the CBN surface under the brazing conditions. The inability of Cr to react with the CBN surface has also been established by the present inventors from the discovery of the fact that from gas phase reaction Cr cannot be deposited on CBN, whereas Cr can be deposited easily by the same CVD process on diamond through formation of chromium carbide at the interface even at 900.degree. C.
A recently published patent (U.S. Pat. No. 4,776,862) discloses a method of fabrication of a monolayer diamond tool by precoating diamond particles with carbide forming metal, e.g., Fe, Cr and Mo, which form carbides of the elements when heat treated. The carbides facilitate wetting of diamond surfaces by the braze alloy. Precoating is done by wetting the abrasive particles with mineral oil or organic binder and then applying fine carbide forming metal powders. The coating process is cumbersome in the sense that it is very difficult to obtain discrete abrasive particles after precoating. This is of particular importance for fabricating a tool having a monolayer configuration. Uniform coating of the entire surface of the abrasive particle is also not very easy when finer grits are used. The fundamental limitation of the process is that elements like Fe, Cr, Mo cannot react with CBN as effectively as they can with diamond producing wettable carbides. CBN is more chemically stable than diamond and instead of carbides, borides or nitrides form if any reaction takes place. Borides and nitrides are definitely less wettable than the respective carbides. Therefore the above disclosed process is so limited in scope that it cannot be used to braze diamond. Thus there remains a need for a low cost practical method for brazing a monolayer of CBN grits to a steel substrate.